It is well known to weld large and relatively thick pieces of steel (or other material) together. For example, pipes are often formed by taking a flat piece of steel and rolling the steel. A longitudinal weld is then made along the ends of the rolled steel, thus forming a section of pipe. A pipe line may be formed by circumferential welding adjacent sections of pipe together. Other applications of welding relatively thick steel (or other material) include ship building, railroad yards, tanker trucks, or other higher strength alloy welding.
When welding such relatively thick steel (or other material) it is generally desirable to preheat the workpiece along the weld path. (As used herein "weld path" refers to the path along which the weld will be made, or has been made.) Pre-heating is used to raise the temperature of the workpiece along the weld path because the filler metal binds to the workpiece better when the weld path is heated, particularly when a high-alloy steel is being welded. Without preheating there is a greater likelihood that the filler metal won't properly bind with the workpiece, and a crack may form. (As used herein "preheating" a weld path means heating the workpiece along the weld path to a pre-weld temperature, or a temperature sufficient to reduce the likelihood of a crack forming along the weld path. Generally, the steel is preheated to about 300.degree. F. prior to welding. Preheating is often used for heating steel having a thickness of more than about 1/2".
Generally, in the prior art, "rose buds" (gas fired flame torches) are used to preheat the steel. The rosebuds are placed along the weld path, typically one rosebud on each side of the weld path, or one covering both sides of the weld path, every 3 to 6 feet. The rosebuds are left in place a relatively long period of time (for example up to two hours for 3" thick steel). After the weld path has been preheated, the rose buds are removed and the weld is performed before the weld path cools. Other methods of preheating a weld path include placing the entire workpiece in an oven (which takes as long as using a rosebud), induction heating, or resistance heating wires. When preheating with these alternatives in the prior art, the heating device is placed at one location on the weld path until that location is heated. Then the weld is performed and the heating device is moved. The preheating has not been done on-the-fly because the heating took much longer than the welding.
Rose buds have also been used in the prior art to dry the weld path prior to welding. In such an application the rose bud will be passed over the weld path immediately prior to welding. However, the rose bud is not applied for a length of time sufficient to preheat the weld.
Unfortunately, the use of rose buds or the other known prior art pre-heating techniques, takes an undesirable long time to heat the weld path. This results in unnecessary down time while the welder is waiting for the weld path to be preheated. In other words a rose bud or other prior art does not heat as the weld path as the weld is being performed, i.e not on-the-fly, so that the welder must wait for the preheating to be completed before welding that portion of the weld path.
Accordingly, a system for preheating weld path is desirable. Preferably, such a system will heat the weld to a pre-weld temperature immediately prior to the weld being performed. Such a system will preferably be able to preheat as fast as the weld can be performed so that as the weld is being performed on one portion of the weld path, a nearby portion is being preheated.